In certain oil producing areas considerable sand, water and like impurities are present in the producing formation in significant amounts in addition to the desired oil. It is not uncommon for the production fluid pumped from such wells to contain more sand and water than oil. Pumping such mixed production fluid presents problems, since the sand especially causes premature wear of seals and other pump parts. Also where considerable sand is present the sand can settle out of the fluid and plug tubing or pack over anchors and thus interfere with retrieval of down-hole tools, pumps, and the like.
Typically in oil wells a casing is fixed into the well. The depth and thickness of the producing formation is known and perforations are blown through the casing and into the formation along a perforated interval from about the top of the producing formation to the bottom. Production fluid made up of oil, sand, water, and the like, depending on the makeup of the formation, flows through the perforations into the well casing and fills the casing to the level of the perforations, and sometimes well up the casing above the formation, depending on the formation pressure.
A pump is then placed in the well and typically is anchored to a fixed location in the casing where the pump intake will remain, during pumping, covered by production fluid so that same may be drawn in by the pump and raised to the surface.
Where production fluid comprises oil, sand, and water, the production fluid standing in the well casing tends to separate vertically such that the production fluid near the surface of the production fluid in the casing contains a greater proportion of oil, which is lighter, while the production fluid lower down in the casing will contain a greater proportion of heavier water and sand, with the proportion of sand, being the heaviest, greater nearer the bottom.
Typically the pump intake is located near the bottom of the perforated interval where sand concentration is heaviest. At times the sand concentration is such that pumping the production fluid with conventional pumps is problematic.
One type of oil production pump uses a piston that moves up and down in a hollow cylinder or pump barrel. The piston is typically moved up and down by a string of rods or tubing, or can also be reciprocated up and down using pressurized fluid. The production fluid can be pushed up the inside of a tubing string located inside the well casing to the surface. If the pump is sealed in the casing the production fluid can be forced into the casing above the pump and so up to the surface.
Such pumps can be either single action, pushing production fluid to the surface on only the upstroke or the downstroke, or dual action, pushing production fluid to the surface on both the upstroke and the downstroke. Where the production fluid contains significant proportions of sand, it is desirable to use a dual action pump in order to keep the production fluid moving up toward the surface. With a single action pump, the production fluid stands still for about half the time, allowing sand to settle out and potentially cause problems. It is preferred to keep the production fluid in motion to reduce the tendency of the sand to settle out.
Such dual action pumps are disclosed in U.S. Pat. No. 4,541,783 to Walling, U.S. Pat. No. 4,871,302 to Clardy et al., U.S. Pat. No. 5,494,102 to Schulte and U.S. Pat. No. 2,933,043 to Furrer, and in United States Patent Application Publication Number 2002/0189805 of Howard. These dual action reciprocating piston pumps use one-way check valves in a variety of configurations to alternately open and close pathways from the production fluid in the casing into the pump barrel on both sides of the piston and then out to the upper casing or tubing and up to the surface. The prior art dual action pumps generally draw production fluid for both the upstroke and the downstroke from substantially the same location at the bottom of the pump.